At the conference, a top Communist Party official called for tighter control of the internet.

US senators asked Apple earlier this year whether the company supported the conference, quoting a critic who said attending the conference made American business officials "complicit actors in the Chinese censorship regime."

The CEOs of the two most valuable technology companies spoke over the weekend at a controversial event in China.

Both Apple CEO Tim Cook and Google CEO Sundar Pichai appeared at China's World Internet Conference, a gathering the Chinese government reportedly uses to promote its strategy of tightly controlling what's available online in the country.

Cook gave a keynote address on Sunday. "The theme of this conference — developing a digital economy for openness and shared benefits — is a vision we at Apple share," Cook said, according to Bloomberg. "We are proud to have worked alongside many of our partners in China to help build a community that will join a common future in cyberspace."

Google's CEO, Sundar Pichai, left, with Xiaomi's founder and CEO, Lei Jun, at a session of the fourth World Internet Conference on Sunday. Reuters

Pichai didn't give a speech, but he did appear on a panel with Xiaomi's founder, Lei Jun. "Technology is giving opportunities at a global scale, driving interconnectedness and cooperation, and I think it's a big trend and I think it's almost irreversible at this point," Pichai said.

One opinion article described Cook as having attended to "kiss the ring." Apple registered $9.8 billion in sales in China for the most recent quarter that figures are available, just under 20% of Apple's overall revenue.

Questions from Congress

Cook arriving at the fourth World Internet Conference. Reuters

The sensitivity of the World Internet Conference is illustrated by a question posed to Apple and Cook by Sens. Ted Cruz and Patrick Leahy earlier this year.

The senators pointedly asked:

"Since 2014, the Chinese government has held a World Internet Conference. Charles Smith, the co-founder of the non-profit censorship monitoring website GreatFire, described foreign guests of the Conference as 'complicit actors in the Chinese censorship regime and are lending legitimacy to Lu Wei, the Cyberspace Administration of China and their heavy-handed approach to Internet governance. They are, in effect, helping to put all Chinese who stand for their constitutional right to free speech behind bars.' What role, if any, has Apple or its employees had with respect to the World Internet Conference in China? Does Apple support the conference?"

"Apple is not a financial sponsor of the conference. Apple has participated in the World Internet Conference in the past, and Apple employees, including our CEO, will participate in the future. We are convinced that continued engagement is the surest way to effect change. We believe in the free flow of information everywhere, and we remain committed to this ongoing effort in all countries in which we operate, across a complicated world."

Less than three weeks later, Cook would be the surprise keynote speaker at the World Internet Conference's opening ceremony.

Read Apple's entire response, which also touches on Chinese censorship of certain apps that can be used to access websites and services that are banned in China: